Periodic Reporting for period 1 - HOCOM (A Transparent Hole Conductor by Combinatorial Techniques for Next-Generation Energy Conversion Devices)
Reporting period: 2019-09-01 to 2021-03-31
This issue severely limits our design options for solar cells and LEDs, it prevents the realization of transparent electronics, and it is a symptom of a gap in our scientific understanding of materials. Advances in hole transparent conductors would likely lead to improvement across all areas of optoelectronics (a key field for renewable energy and energy efficiency). Answering the scientific question “what makes a good transparent hole conductor?” would also be likely to trigger new fields and opportunities in materials science.
The main goal of HOCOM is to experimentally evaluate certain phosphide materials as potential transparent hole conductors. Synthesis of these candidate materials in thin-film form (as relevant for optoelectronic devices) is made possible by a unique deposition setup at the National Renewable Energy Laboratory (NREL, USA) dedicated to phosphides. Detailed characterization of the most interesting phosphides will be performed at the Helmholtz Zentrum Berlin (HZB, Germany).
In the 16-month outgoing phase at NREL, we applied combinatorial research techniques to rapidly evaluate the potential of selected phosphide thin films as transparent hole conductors. The growth chamber was a unique sputter system equipped with diluted phosphine, which allowed us to grow phosphide films by reactive sputtering of metallic targets. The two main materials we investigated were BP (boron phosphide) and CaCuP. For the case of BP, we were able to obtain the correct stoichiometric ratio between B and P, but all the grown BP film were amorphous and had a very low conductivity. Post-annealing these films allowed us to obtain the desired BP crystal structure and more encouraging conductivities. Under specific growth and annealing conditions, the BP films exhibited hole conduction as desired and were at least partially transparent. However, the performance figure of merit was rather low. As a second track, we investigated reactive sputtering of CaCuP. This compound could be indeed synthesized in one step at an appropriate temperature. We found very high hole conductivity under specific process conditions. The optical transparency was less than optimal, in disagreement with theoretical expectations. We are currently working with computational collaborators to understand this discrepancy. Nevertheless, CaCuP still exhibits a reasonably high figure of merit and a very remarkably high conductivity for a p-type semiconductor without external doping. Thus, our results warrant further investigation of this exotic compound.
In the second part of HOCOM, we plan to further characterize BP and CaCuP using the advanced characterization setups available at HZB. In particular, we will try to identify the dominant defects responsible for hole conduction by various spectroscopy techniques. We will also investigate the band alignment of BP and CaCuP with common semiconductors, to evaluate their applicability in a real device. In case of a positive answer, we will fabricate a simple diode device to show that phosphide hole conductors are a feasible option for next-generation optoelectronic devices.